The present invention generally relates to magnetic recording mediums, and more particularly to a magnetic recording medium having both lubricity and corrosion resistance.
Recently, as a magnetic recording medium for high density recording, a metal thin film type magnetic recording medium has been proposed and reduced to practice. Such a metal thin film type magnetic recording medium comprises a base and a magnetic layer formed thereon. The magnetic layer is made of a magnetic material such as Co, Co--Ni, Co--P, and Co--Ni--P, and the magnetic layer is formed on the base by a method such as sputtering, electroplating, and electroless plating.
Generally, when carrying out a recording or reproduction by use of the type of magnetic recording medium described above, a magnetic head is slightly floated from the magnetic recording medium due to an air film generated by an airflow which occurs as the magnetic recording medium rotates at a high rotational speed. However, the magnetic head makes contact with the magnetic recording medium when the magnetic recording medium starts to rotate until a steady rotational speed is reached, and also when the rotational speed is reduced so as to stop the rotation of the magnetic recording medium. For this reason, the magnetic layer of the magnetic recording medium wears out due to the friction between the magnetic head and the magnetic layer.
Accordingly, in order to prevent premature wear of the magnetic layer due to the friction, a thin film of carbon having lubricity and a small coefficient of friction is conventionally formed on the magnetic layer as previously proposed in a Japanese Published Patent Application No. 33521/1974 and a Japanese Laid-Open Patent Application No. 41524/1981, for example. The magnetic layer is protected from the magnetic head by the provision of the thin carbon film. Further, because carbon has lubricity, the coefficient of friction with respect to the magnetic head is small, and as a result, an excessively large friction is prevented from being introduced at the surfaces of the magnetic recording medium and the magnetic head.
However, it has been found that under a dew atmosphere, rust occurs and corrosion takes place on the magnetic layer of the magnetic recording medium having no carbon film on the magnetic layer and even on the magnetic layer of the magnetic recording medium having the thin carbon film formed on the magnetic layer. The present inventors have studied this phenomenon and found that the thin carbon film itself has an internal stress, and that cracks are formed in the thin carbon film due to a slight change in the ambient condition or even under a steady condition in an extreme case. When cracks are formed in the thin carbon film, moisture enters through the cracks and the corrosion takes place on the magnetic layer.
In order to improve the corrosion resistance of the magnetic layer, a film made of SiO.sub.2 was experimentally formed on the magnetic layer as a corrosion resisting layer. Further, a lubricant layer made of carbon was additionally formed on the corrosion resisting layer so as to improve the lubricity. However, even in this case, it was found that cracks are formed not only in the carbon lubricant layer but also in the corrosion resisting layer due to the internal stress of the lubricant layer, and that the corrosion resistance is poor.